Heretofore, there has been known a digital coherent receiver that converts information of both of the amplitude and phase of light to electric currents and decodes data by a digital processing. The digital coherent receiver uses, for example, an adaptive equalizer (AEQ) configured to perform adaptive equalization processing for dealing with temporal variation in propagation characteristic of the optical transmission path. Also, in the optical communication and radio communication, there has been also known a technique of compensating an offset (frequency offset) between an oscillation frequency on the transmission side and an oscillation frequency on the reception side (for example, see Japanese Laid-open Patent Publication Nos. 2012-244255, 2003-198651, 2013-123161, and 2014-060708).
However, the technique described above has a problem that when the transmission path of the optical signal is long, for example, noise generated in the optical signal is so large that it is difficult to estimate the frequency offset with high accuracy.
For this reason, the technique described above is unable to achieve high accuracies in channel estimation and setting of the AEQ based on the channel estimation or the result of the channel estimation, and consequently has a problem that the distortion caused in the optical signal by transmission path characteristics may not be compensated by the AEQ with high accuracy.
According to one aspect, it is an object of the embodiment to provide an adaptive equalization circuit, a digital coherent receiver, and an adaptive equalization method capable of obtaining a highly accurate estimation value of a frequency offset by reducing effects in the noise.